Semiconductor chips and other microelectronic elements typically have contacts which must be connected to external circuitry, such as the circuitry of a supporting substrate or circuit panel. Various processes and components for making these connections have been provided heretofore. For example, in a wire bonded assembly the chip is physically mounted on a substrate and individual fine wires are connected between the contacts of the chips and contact pads on the substrate. In tape automated bonding or "TAB" processes, a dielectric supporting tape such as a thin foil of a polymer is provided with a hole slightly larger than the chip. An array of metallic leads is provided on one surface of the dielectric tape. These leads extend inwardly from around the hole so that an inner end of each lead projects inwardly beyond the edge of the hole. These ends are arranged side by side at spacings corresponding to the spacings of the contacts on the chip. The inner ends of the leads are bonded to the contacts on the chip, whereas outer ends of the leads are attached to contact pads on the substrate. In the "beam lead" process, the chip is provided with leads extending from contacts on the chip outwardly beyond the edges of the chip. The chip is positioned on the substrate so that the outer ends of the leads lie over the appropriate contact pads of the substrate and the leads are bonded to the contact pads.
The rapid evolution of the semiconductor art has created a need for progressively greater numbers of contacts and leads in a given amount of space. An individual chip may require hundreds or even thousands of contacts and leads. For example, a complex semiconductor chip in current practice may have rows of contacts spaced apart from one another at center-to-center distances of 0.5 mm or less. These distances are decreasing progressively with continued progress in the semiconductor art. With such closely spaced contacts, the leads connected to the chip contacts such as the wires in wire bonding and the leads used in the TAB and beam lead processes must be extremely fine structures, typically less than 0.1 mm wide.
As disclosed, for example, in U.S. Pat. Nos. 5,489,749 and 5,536,909 and in PCT Published International Application WO 94/03036, published Feb. 3, 1994, all of which are incorporated by reference herein, a component for mounting a semiconductor chip may include a support structure such as a polymeric film defining one or more slots and one or more leads extending across such slots. The support structure typically has terminals on it for connection to a circuit panel or external circuitry. Each lead has a connection section with a first end permanently secured to the supporting structure on one side of the slot and a second end remote from the first end. The first ends of the lead connection sections typically are connected to the terminals. The second end of each connection section is releasably secured to the opposite side of the slot. For example, each lead may include a frangible section connecting the second end of the connection section to the support structure. Thus, each lead bridges across the slot in the support structure.
Each lead is then engaged by a bonding tool which enters the slot and forces each lead downwardly into engagement with the appropriate contact of the chip. This motion releases the lead from the side of the slot opposite the terminals. For example, the frangible section of the lead may be severed by the downward movement of the lead. Preferably, each lead is guided by the bonding tool during this operation. When the leads are bonded to the contacts of the chip, the terminals on the supporting structure are electrically connected to the chip. Thus, the chip can be connected to external circuitry by attaching the terminals on the supporting structure to a larger substrate, as by solder bonding the terminals to the larger substrate. The bonded leads typically provide flexible interconnections between the contacts of the chip and the terminals and thus allow compensation for thermal expansion and contraction of the chip and substrate. Components and bonding methods as disclosed in the '749 and '909 patents and in the '036 International Publication provide rugged, compact and economical chip mountings, and offer numerous advantages. Still further improvements and enhancements to such chip mountings are disclosed in U.S. Pat. No. 5,398,863 and U.S. Pat. No. 5,491,302, the disclosures of which are hereby also incorporated by reference herein. Bonding tools useful in attaching leads according to the '794 patent and '036 publication and for other purposes are disclosed in U.S. Pat. No. 5,390,844 and U.S. patent application Ser. No. 08/630,375, filed Apr. 10, 1996, now U.S. Pat. No. 5,868,301, the disclosures of which are also incorporated by reference herein.
U.S. patent application Ser. No. 08/491,809, filed Jun. 16, 1995, now U.S. Pat. No. 5,597,470, and Ser. No. 08/374,559, filed May 8, 1995, now U.S. Pat. No. 5,915,752, disclose metallic buses that are formed on the support structure, extending parallel to the slot and perpendicular to the leads, on the side of the slot opposite the terminals. The leads are each electrically and mechanically connected to the bus. The metallic bus is used for providing current to the leads and terminals during a plating process. For example, the terminals and leads may be plated with a bondable cover material such a gold, silver, platinum, palladium and alloys thereof, using the bus to provide current during the plating process. After the plating process is completed, the leads are mechanically severed from the plating buses during the process of bonding the leads to the chip. As the lead is displaced downward through the slot and into contact with a terminal on the chip, a frangible portion of the lead near the side of the slot where the leads connect to the plating buses is broken, disconnecting the lead from the support structure on that side of the slot and electrically severing the lead from the plating bus. The lead may be displaced downward using a specially configured bonding tool.
The '809 application discloses a preferred embodiment in which the leads are severed from the plating bus one at a time. In this process, a bonding tool displaces each lead downward and bonds it to a terminal on the semiconductor chip before advancing to the next lead. In this way, each lead is connected to either the plating bus or the semiconductor chip except the lead actually undergoing the bonding process.
In some cases, the manufacturer performing the lead bonding operation does not wish to detach the lead from the supporting structure, as by breaking a frangible section of the lead, during the bonding operation. In this case, it would be desirable to provide cantilevered leads extending partially across the bonding slots, with free ends available for downward displacement and bonding. However, it has not been practical heretofore to provide such free ends on leads arranged to form a "fan-in" chip mounting, in which the contacts on the chip are arranged at the periphery of the chip and the terminals mounted on a dielectric structure and connected to the leads are disposed overlie the center of the chip.